Abstract Increasing evidence indicates that proper host-microbe interactions in the gastrointestinal tract are crucial for human overall health and well-being. Dysregulated host responses to gut microbiota are major contributors to allergic, immunologic, and infectious diseases, resulting in very high economic and quality of life burdens. Yet, prevention or early diagnosis and treatment of these diseases remain a challenge due to our limited knowledge of the mechanisms underlying host-microbe homeostasis. Our long-term goal is to gain a thorough understanding of the molecular mechanisms by which host-microbe homeostasis is precisely controlled to improve the diagnosis and the treatment of human diseases caused by dysregulated host-microbe interactions. The objective of this application is to determine roles of hnRNPI (heterogeneous nuclear ribonucleoprotein I), which we recently identified as a novel regulator of neonatal immune tolerance, in downregulating TLR signaling and inducing intestinal immune tolerance. Development of intestinal immune tolerance is a fundamentally important step in establishing host-microbe homeostasis, and is associated with transient suppression of TLR signaling. Transient suppression of TLR signaling activity has been reported to be critical for microbe colonization in the neonatal intestine and development of endotoxin tolerance in adulthood. The precise mechanisms underlying TLR signaling suppression remain unclear, but have been shown to correlate with proteolytic degradation of IRAK1, a key component of TLR signaling. Using a mouse model in which hnRNPI is ablated in intestinal epithelial cells, we demonstrate that hnRNPI downregulates protein level of IRAK1 and suppresses activity of TLR ?mediated NF-?B signaling in the neonatal colon. To better understand hnRNPI-mediated TLR signaling suppression, here we propose to investigate whether hnRNPI controls neonatal intestinal immune tolerance through downregulating MyD88-mediated TLR signaling. Moreover, we propose to investigate the role of hnRNPI in the development of endotoxin tolerance in adulthood. Completion of the proposed studies will build a solid foundation for our future in-depth mechanistic studies on hnRNPI- dependent TLR signaling regulation, which will likely uncover novel mechanisms important for the control of intestinal innate immunity.